One conventional method of varying the output power of a laser is to vary its input voltage or current. This voltage or current can be controlled by a feedback circuit connected to a detector that monitors the output power. In this way the output power can be kept constant, as the efficiency of the laser deteriorates with time. See, for example, U.S. Pat. No. 4,611,270 issued Sept. 9, 1986 to G. K. Klauminzer et al. This general method has the disadvantage, however, that variation of the input voltage has a tendency to change other parameters of the laser output, that it is not desired to change, such as pulse length, pulse shape, beam size, and beam homogeneity. Also, in the case of many pulsed lasers including excimer lasers there may be more than one storage means and hence more than one voltage that would require to be varied. See, for example T. J. McKee et al, "A High-Power Long Pulse Excimer Laser", IEEE Photonic Technology Letters Vol. 1, No. 3, pp. 59-61 (1989).
There are also some lasers, including excimer lasers, in which variation of the input voltage affords only a very narrow range of output power values.
In addition to these situations in which it is desired to maintain the output power constant, i.e. to restore it to an original value as the laser's efficiency deteriorates with time, there are also occasions in which it is desired to reduce the output power to a lower level for a particular use.
For these and other reasons, the art has developed various devices for the attenuation of a light beam. One such device is an absorber that is located in the beam. This approach is satisfactory for low power beams, but presents overheating problems when the power of the beam is high. In this latter situation it is better to use a partial reflector for reflecting some of the light. If desired, the reflectivity of the reflector can be varied by various means.
One such proposal for varying the reflectivity has been suggested by D. A. B. Miller et al in "Variable attenuator for Gaussian lasers beams", published in Applied Optics, Vol. 17, No. 23, Dec. 1, 1978, pp 3804-3808. This proposal employs multilayer dielectric mirrors to form an interference filter that is varied in thickness across the width of a substrate, i.e. is slightly wedge shaped. This device forms an attenuator that varies its transmittance as it is moved transversely across a laser beam. The difficulty with this system is that the transmittance is different across the transverse dimension of the beam. This was not a problem in the Miller et al proposal, because the beam had been given a very small diameter by being passed through a 200 .mu.m diameter pinhole to act as a spatial filter, but is unsatisfactory for wider beams.
In the absence of a spatial filter, a typical laser beam width is much greater than 200 .mu.m. A typical beam of a CW gas laser is round in cross-section with a diameter anywhere from about 0.1 mm to 10 mm. For pulsed solid state lasers, the beams are also usually round with diameters between 5 mm and 20 mm. For pulsed gas lasers, the beams are usually square or rectangular. For example the beam of a CO.sub.2 laser could be from 5.times.5 mm to 50.times.50 mm. The beams of excimers lasers are typically 5.times.20 mm, but they can be square, e.g. 30.times.30 mm. One excimer laser has a round beam of diameter 1 mm or so.
It is also known to use multilayer dielectric coatings as static attenuators in very high power lasers. The user selects an attenuator from a set of graded multilayer dielectric coated substrates all designed for a specific wavelength and having respective reflectivities of 10%, 20%, 50%, 90% and 99%, for example. These attenuators will be so designed that their reflectivity is relatively insensitive to the angle of incidence of the beam, at least between 0.degree. and about 20.degree.. Once selected and installed, the attenuator will be mounted in a fixed position, usually at a slight angle to the beam, e.g. 5.degree., to avoid reflection back into the optical cavity of the laser.